On Wed, Nov 11, 2020 at 3:31 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 11/9/20 5:52 AM, Oscar Salvador wrote:
> > On Sun, Nov 08, 2020 at 10:10:55PM +0800, Muchun Song wrote:
> >> The purpose of introducing HUGETLB_PAGE_FREE_VMEMMAP is to configure
> >> whether to enable the feature of freeing unused vmemmap associated
> >> with HugeTLB pages. Now only support x86.
> >>
> >> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> >> ---
> >> arch/x86/mm/init_64.c | 2 +-
> >> fs/Kconfig | 16 ++++++++++++++++
> >> mm/bootmem_info.c | 3 +--
> >> 3 files changed, 18 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> >> index 0a45f062826e..0435bee2e172 100644
> >> --- a/arch/x86/mm/init_64.c
> >> +++ b/arch/x86/mm/init_64.c
> >> @@ -1225,7 +1225,7 @@ static struct kcore_list kcore_vsyscall;
> >>
> >> static void __init register_page_bootmem_info(void)
> >> {
> >> -#ifdef CONFIG_NUMA
> >> +#if defined(CONFIG_NUMA) || defined(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP)
> >> int i;
> >>
> >> for_each_online_node(i)
> >> diff --git a/fs/Kconfig b/fs/Kconfig
> >> index 976e8b9033c4..21b8d39a9715 100644
> >> --- a/fs/Kconfig
> >> +++ b/fs/Kconfig
> >> @@ -245,6 +245,22 @@ config HUGETLBFS
> >> config HUGETLB_PAGE
> >> def_bool HUGETLBFS
> >>
> >> +config HUGETLB_PAGE_FREE_VMEMMAP
> >> + bool "Free unused vmemmap associated with HugeTLB pages"
> >> + default y
> >> + depends on X86
> >> + depends on HUGETLB_PAGE
> >> + depends on SPARSEMEM_VMEMMAP
> >> + depends on HAVE_BOOTMEM_INFO_NODE
> >> + help
> >> + There are many struct page structures associated with each HugeTLB
> >> + page. But we only use a few struct page structures. In this case,
> >> + it wastes some memory. It is better to free the unused struct page
> >> + structures to buddy system which can save some memory. For
> >> + architectures that support it, say Y here.
> >> +
> >> + If unsure, say N.
> >
> > I am not sure the above is useful for someone who needs to decide
> > whether he needs/wants to enable this or not.
> > I think the above fits better in a Documentation part.
> >
> > I suck at this, but what about the following, or something along those
> > lines?
> >
> > "
> > When using SPARSEMEM_VMEMMAP, the system can save up some memory
> > from pre-allocated HugeTLB pages when they are not used.
> > 6 pages per 2MB HugeTLB page and 4095 per 1GB HugeTLB page.
> > When the pages are going to be used or freed up, the vmemmap
> > array representing that range needs to be remapped again and
> > the pages we discarded earlier need to be rellocated again.
> > Therefore, this is a trade-off between saving memory and
> > increasing time in allocation/free path.
> > "
> >
> > It would be also great to point out that this might be a
> > trade-off between saving up memory and increasing the cost
> > of certain operations on allocation/free path.
> > That is why I mentioned it there.
>
> Yes, this is somewhat a trade-off.
>
> As a config option, this is something that would likely be decided by
> distros. I almost hate to suggest this, but is it something that an
> end user would want to decide? Is this something that perhaps should
> be a boot/kernel command line option?
Yeah, it already has a boot/kernel command line option named
"hugetlb_free_vmemmap". We can refer to
[PATCH v3 18/21] mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap
Thanks :)
>
> --
> Mike Kravetz
--
Yours,
Muchun
